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Full text of "Text Book Of Mechanical Engineering"

604

Specific Heats of Steam.

Let us heat a gas under a constant pressure P,
being increased from Vl to V2 and the temperature   ri**1
T-J to r2 absolute : then

External work * P (V2 - Vj) = c (r2 - r- , >
Total heat expended = spec, ht x rise in tern } **

' = Kp (r2 - TJ)
and Internal work = Total - External

= Kp (r, - T-J) - * (r2 -    *-» I

But, when a gas is heated at constant 'volume,
work is done.

Kv (r2 —

KP (r, - rj -

C.

and Kp - K

Note that internal work is always Kv (final
temp.), and may therefore be positive, negative, or

Specific Heats of Superheated Steam.—By
Kp = 370*56 foot Ibs., and as steam a few degrees al>**v*
tion point is a practically perfect gas, Kp will be a regal        4
Further, if we are heating at constant pressure,

For steam PVS = csr               For air PVa =» <r^ ur

Now the ratio of specific volumes —

"62.2.

622

and

Then Kp - Kv » 85*5 and Kv
Finally y = —£

•622     '622
370*56-85*51

- 37o:56 =

~ 285*06     •=-^:





Expansion Curves and their Areas. — The

co-ordinatdng -Boyle?s law, has been shewn at Fig,

other expansion curve, as these are called, has the forr» it H

the exponent n changing with the substance*    Now

area, Fig. 612, shews the work done during expansion^   mi